Heretofore, various methods of forming a ceramic porous membrane on porous base members have been known.
For example, a hot coating process is known (see Non-Patent Document 1). This is a method of rubbing a tube base member with a cloth containing a silica sol to apply the silica sol thereto and thereby form a porous membrane on an outer surface of the tube base member heated at about 200° C. In addition, a method of forming a sol into a thin membrane on a substrate of a ceramic or the like by a dipping process is also known (see Non-Patent Document 2). This is a method of dipping the substrate into a dipping liquid and then forming the membrane thereon while pulling up the substrate.
A method of forming a porous membrane on an inner surface of a porous base member having a tubular shape or a cylindrical lotus root-like monolith shape by filtering membrane formation is also known (see Patent Documents 1, 2). The outer surface of the porous base member is held at a pressure lower than that of an inner surface thereof which comes in contact with a sol liquid to form the membrane on the inner surface of the porous base member. A membrane forming method of successively reducing pore diameters to laminate membranes is also known (see Patent Document 3).
However, the hot coating process has a problem that the membrane cannot uniformly be formed on the whole base surface, and the process cannot be applied to the inside of the monolith base having the tubular or cylindrical lotus root-like shape. In the filtering membrane formation process, during drying of the formed membrane, a solvent present in a base member pore sometimes flows out on a membrane side to cause membrane peeling. As a result, there is a problem that a defect is generated in the porous membrane formed on the fired base surface.
Moreover, in the dipping process, the problem generated in the filtering membrane formation process is not generated, but it is difficult to pull up a submerged long and large base member. Therefore, there is difficulty in applying the process to a base member having a large membrane area. The pores having a so-called coarse pore diameter larger than an average pore diameter tend to increase. To prevent this problem, the membrane needs to be thickened.
Furthermore, in a method in which a pressure difference is created between the side of an inner peripheral surface and the side of an outer peripheral surface of the tubular porous base member before the base member comes in contact with the sol liquid, a phenomenon where the solvent filled in the base member pores during the membrane formation flows toward the membrane during the drying and the membrane of nano-level particles peels occurs in the same manner as in the filtering membrane formation process. This results in a problem that the defects are generated in the porous membrane formed on the fired base surface.
Non-Patent Document 1: M. Asaeda et al., “Separation and Purification Technology” 25 (2001) 151 to 159;
Non-Patent Document 2: “Science of Sol-Gel Process” authored by Sumio Sakuhana, Agnes Shofu-sha, pages 85 to 95, 1988;
Patent Document 1: Japanese Patent Application Laid-Open No. 61-238315
Patent Document 2: Japanese Patent Application Laid-Open No. 3-267129; and
Patent Document 3: Japanese Patent Application Laid-Open No. 6-198148.